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Subject: Re: [PATCH 2.6.13-rc6-rt9]  PI aware dynamic priority adjustment
From: Thomas Gleixner <tglx@linutronix.de>
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To: george@mvista.com
Cc: Ingo Molnar <mingo@elte.hu>, Roland McGrath <roland@redhat.com>,
       Oleg Nesterov <oleg@tv-sign.ru>, linux-kernel@vger.kernel.org,
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       "Paul E. McKenney" <paulmck@us.ibm.com>
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Date: Sat, 20 Aug 2005 02:36:12 +0200
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George,

On Fri, 2005-08-19 at 17:19 -0700, George Anzinger wrote:
> > 2. Drift of cyclic timers (armed by set_timer()):
> > 
> > Due to rounding errors and the drift adjustment code, the fixed
> > increment which is precalculated when the timer is set up and added on
> > rearm, I see creeping deviation from the timeline. 
> > 
> > I have a patch lined up to base the rearm on human (nsac) units, so this
> > effect will go away. But this is waste of time until (1.) is not solved.
> > 
> > George ???
> 
> Could I (we) see what you have in mind?

Nothing which applies clean at the moment and I have no access to the
box where the patch floats around.

It's simply explained.

Current code:

set_timer()
	calc interval->jiffies / interval->arch_cycles;
	based on it.interval

rearm()
	timer->expires += interval->jiffies;
	timer->arch_cycle_expires += interval->arch_cycles;
	normalize(timer);

Patched code:

set_timer()
	timer.interval = it.interval; 
	timer.next_expire = it.value; 
	both stored as timespec

rearm()
	next_expire += interval;
	calc timer->expires/arch_cycle_expires;
	
So on each rearm we eliminate the rounding errors and take the drift
adjustment into account.

It adds some calculation overhead to each rearm, but ....

tglx